This invention relates to a device for metering free-flowing to poor-flow dry powders and powder mixtures for use in powder or liquid coating compositions.
Various possibilities are known for metering bulk solids in general and powder coatings or powder coating components in particular, e.g., feed screws, vibrating feed units, metering gates, metering valves.
Feed screws are able to convey the material to be transported or metered in any configuration and gradient but with the disadvantage that the technological properties of the metered material may be affected considerably by the shear forces of the screw. A further disadvantage is that, particularly with highly free-flowing products, accurate metering may be guaranteed only at great expense. Metering of bulk solids using vibrating feed units takes place by vibration. With very fine bulk solids, lumps or briquettes may be formed. Metering gates which may be operated electrically, pneumatically or hydraulically permit a relatively high degree of metering accuracy but they can be used only with very free-flowing materials. Metering valves, e.g., in the form of metering cones, are technically complex and may lead to caking and lumps on the valve or on the product discharge.
The metering units mentioned are therefore unsuitable in terms of metering accuracy, metering speed and discharge behaviour, particularly for poor-flow powders used in coating compositions, such as pigments and additives.
Metering devices based on screen metering with a vibration drive are also known. For example, DE-AS 1619823 describes a process for the preparation of crystals by flame melting of materials in powder form, wherein a vibrating screen is used as the conveyor unit. The purpose of this vibrating screen is to feed a defined amount of the powder to the flame melting device by vibration. The amount of powder is fed to the vibrating screen in a complex manner by way of a tapping mechanism on a powder storage vessel by means of a striking device arranged on the vessel. The amount of powder situated on the vibrating screen should always be kept constant and small in order to prevent briquetting of the fine powder. To this end, expensive monitoring devices are required.
According to DE-A 3038269, a vibrating screen is integrated in a storage vessel for seed dressing agent in powder form. This storage vessel with an integrated vibrating screen is mounted on the relevant collecting or metering vessel and is used to fill the vessel by metering by means of the vibrating screen. A disadvantage of this procedure is that the storage vessel has to be attached in a complex manner to the collecting or metering vessel by way of additional fixing devices. The screen itself can be changed only in a very labour-intensive and time-consuming manner, if at all. It is scarcely possible to use relatively large storage vessels in view of the size and weight to be fixed to the collecting vessels.
The present invention provides an easy-to-handle device for metering dry powder materials, which device permits a metering process which does not depend on complex designs, with high accuracy and speed of metering at the metering site, regardless of the form of the metered material and without loss of quality of the material to be metered.
The device for metering dry powders comprises a storage vessel having an outlet, and a free-swinging receiving vessel having a vibratable metering base, wherein said receiving vessel is positioned at the outlet of said storage vessel and wherein the distance between the outlet of the storage vessel and the metering base of the receiving vessel can be dimensioned as a function of the bulk solids properties of the powder material, and the metering base of the receiving vessel is vibrated during metering.